"People say, 'Was your wedding the most special day?' And I say, 'well, actually it wasn't. It was the coronation,'" Glenconner said.

She was only 19 at the time, and still proudly keeps her dress at home. And while she and Elizabeth were childhood friends, that wasn't the only reason she was picked.

"We were chosen 'cause we had good figures. We were the right height," she said.

"Do you really think that's true?" D'Agata asked.

"Of course it was. It was like a production. It was like making a film," she said.

In "The Coronation," the queen shares for the first time her memories of that day, including the bumpy carriage ride.

"Horrible. It's not meant to be traveling in at all. It's just not -- it's only sprung on leather," Queen Elizabeth recalls in the documentary. "Not very comfortable."

Although documentary maker Alastair Bruce says he was a little nervous, he said Her Majesty made him feel welcome.

"I think people are always surprised by her absolute humility. She is not sitting there, waiting for people to bow and curtsy. She's not bothered about that at all. She is keen to put people at their ease. And she does so with great gentleness," Bruce said.

Even so, Lady Glennconner admits to being a little starstruck when it came time to play her role.

"There was the queen. We hadn't ever seen her in her coronation dress. And she looked absolutely ravishing," she said. "We were all waiting there. And she just turned round, and she said, 'Ready, girls?' And we sort of nodded and off we went."

And nothing will ever compare to the moment she joined the new monarch on the balcony.

"You could actually feel, physically feel, the sort of love and the noise coming at you. It was the most extraordinary feeling. And they were mad about her. They were so happy," Glennconner said.

They're still mad about her. In the documentary, the queen joked about how heavy that crown was. She said she had to keep her head straight, because if she leaned forward it would break her neck and the crown would fall off.